1. Field of the Invention
This invention generally relates to a method and an apparatus for inspecting an optical disc. This invention particularly relates to a method and an apparatus for checking whether the difference between track positions, which should align with each other, on respective adjacent recording layers of a multi-layer optical disc is within a prescribed allowable error range.
2. Description of the Related Art
There is a prior-art method of inspecting a DVD-R having two recording layers. The prior-art method utilizes a microscope. The details of an essential part of the prior-art method are as follows.
The two recording layers of a DVD-R are called the “0” layer and the “1” layer, respectively. Signal recording with a prescribed track width is performed on each of the “0” layer and the“1” layer. As a result, there occurs a signal recorded area and a signal unrecorded area or areas in each of the “0” layer and the “1” layer. Three points on the circumferential boundary between the signal recorded area and the signal unrecorded area in the “0” layer are two-dimensionally observed by using a microscope. The track center position and the track diameter concerning the “0” layer are determined from the three observed points. Specifically, first and second points among the three observed points are connected by a first straight line segment, and the remaining point and one of the first and second points among the three observed points are connected by a second straight line segment. A first perpendicular bisector of the first straight line segment is constructed. A second perpendicular bisector of the second straight line segment is constructed. The point of the intersection of the first and second perpendicular bisectors is the track center position concerning the “0” layer. Doubling the distance between the track center position and one of the three observed points equals the track diameter concerning the “0” layer. Similarly, the track center position and the track diameter concerning the “1” layer are determined. The distance between the track center position concerning the “0” layer and that concerning the “1” layer is a center deviation quantity (a center error quantity or an eccentricity) Rd whose adverse effect appears when the optical disc rotates.
A maximum relative error value Dmax and a minimum relative error value Dmin are prescribed as error-related requirements for the manufacture of an optical disc. In the case where the start address in the “0” layer is FFCFFFh in hexadecimal notation, a track position on the “1” layer which should align or radially coincide with the position at the start address in the “0” layer is at an address of 003000h plus an Ax value of 0.258 mm. The Ax value is a prescribed radial track difference between the “0” layer and the “1” layer. Regarding the manufacture of an optical disc, an allowable error range for the above-indicated track position on the “1” layer is between +0.168 mm and +0.348 mm in terms of the Ax value (+0.258 mm).
The maximum relative error value Dmax and the minimum relative error value Dmin are calculated from the center deviation quantity Rd according to the following equations.Dmax=(DL0−DL1)/2+Rd Dmin=(DL0−DL1)/2−Rd where DL0 denotes the track diameter concerning the “0” layer, and DL1 denotes the track diameter concerning the “1” layer. The maximum relative error value Dmax and the minimum relative error value Dmin are required to satisfy specified error-range conditions of a “1” layer track position relative to an ECC block address in the “0” layer track. It should be noted that the address (003000h) of the track position on the “1” layer which corresponds to the start address (FFCFFFh) in the “0” layer is obtained through the bit inversion of the start address. The track position on the “1” layer which should align or radially coincide with the position at the start address (FFCFFFh) in the “0” layer results from radially outwardly shifting the position at the start address by a distance equal to the Ax value.
Japanese patent application publication number 11-203692/1999 discloses an optical head including a semiconductor laser, a main photodetector, a defect sensing photodetector, a collimator lens, an objective lens, and a mirror. Main portions of a laser beam emitted from the semiconductor laser are focused into spots on an optical disc through the collimator lens and the objective lens. The main photodetector receives reflected laser light from the optical disc which originates from the main portions of the laser beam. The main photodetector converts the received laser light into a reproduced signal, a focusing error signal, and a tracking error signal. Tracking control of the laser beam with respect to the optical disc is implemented in response to the tracking error signal. The laser beam emitted from the semiconductor laser has a defect detecting portion traveling along a path outside the field of view of the collimator lens. The mirror directs the defect detecting portion of the laser beam to the optical disc. The defect detecting portion of the laser beam is focused on a position in the optical disc which precedes the spots of the main portions of the laser beam as viewed in a direction of rotation of the optical disc. The defect sensing photodetector receives reflected laser light from the optical disc which originates from the defect detecting portion of the laser beam. The defect sensing photodetector converts the received laser light into a corresponding electric signal. The level of the electric signal is monitored. A defect in the optical disc is detected from a variation in the monitored level of the electric signal. In the case where a defect in the optical disc is detected, the tracking control is suspended for a prescribed time interval while the objective lens remains fixed. Accordingly, the defect in the optical disc is prevented from causing a tracking offset which would disturb a tracking servomechanism.
Japanese patent application publication number 10-293926/1998 discloses a DVD-R which has a land track formed with pre-pits representing on-disc address information called LPP (land pre-pit) address information. Before the recording of information data on the DVD-R, the LPP address information is detected therefrom. During the recording of information data, the currently-accessed position on the DVD-R is controlled according to the detected LPP address information.
It is known that a DVD-RW having multiple recording layers is formed with land pre-pits (LPP) representing on-disc address information.